This invention relates to a read channel for a magnetic record in the form of digital information and more particularly to a read pulse generator capable of correct detection of data signals.
The read channel for a magnetic record is typically constructed as exemplified in FIG. 1. Thus, a signal represented by reference characters 1a and 1b (more strictly, a positive-going portion is represented by 1a and a negative-going portion is represented by 1b) is a reproduction signal from a magnetic head 1, and this reproduction signal is amplified by a pre-amplifier 2 into a signal 2a, 2b which in turn is passed through an automatic gain control (AGC) circuit 3. An output signal from the AGC circuit 3, as represented by 3a and 3b, is shaped by an equalizer 4 into a signal represented by 4a and 4b. This signal 4a, 4b is, on the one hand, differentiated by a differentiation circuit 5 to produce a signal 5a, 5b that is passed through a low-pass filter 6 to provide a differentiated signal 6a, 6b which in turn is supplied to a read pulse generator 8. On the other hand, the signal 4a, 4b branches to a low-pass filter 7 and a non-differentiation or reproduction signal 7a, 7b therefrom is also supplied to the read pulse generator 8. The read pulse generator 8 derives from pulses representative of the differentiated signal only pulse edges which correctly reflect peaks or correct data portions of the non-differentiation signal. These pulse edges are used to form a read pulse signal 8a. The read pulse signal 8a directly connects to a discriminator 10 and at the same branches to a variable frequency oscillator (VFO) 9. The VFO 9 then produces a detection window signal 9a which is supplied to the discriminator 10. By using the read pulse signal 8a and the detection window signal 9a, the discriminator 10 performs phase discrimination to form a read data signal 10a which is sent as reproduction information to a host controller.
One example of this type of read channel for magnetic record is disclosed in U.S. Pat. No. 4,081,756 to Robert Price et al. A typical example of a read pulse generator used for the read channel for magnetic recording is shown in FIG. 2 and a time chart illustrative of signal waveforms appearing in the read pulse generator is shown in FIG. 3. The differentiated signal 6a, 6b is fed to a limiter circuit 11 in order for its zero-cross points to be detected. An output pulse signal from the limiter circuit 11, as represented by 11a and 11b, may probably contain zero-cross signal pulse edges indicated at 11A in FIG. 3 which do not correspond to any correct data portion of the non-differentiation signal 7a, 7b. This is because at portions of the signal 7a, 7b where no magnetization inversion occurs, there occurs no change in magnetization and the non-differentiation signal indicative of the head reproducing signal becomes substantially zero and flattened and is differentiated to generate erroneous zero-cross signal pulse edges. To eliminate the erroneous zero-cross signal pulse edges 11A, gate signals 14a and 16a corresponding to peaks or correct data portions of the non-differentiation signal 7a, 7b are generated. Correct pulse edges of zero-cross signal representative of correct data portions can be extracted using the gate signals 14a and 16a. More specifically, a set/reset flip-flop 18 and D-type flip-flops 19 and 20 are used in such a manner that only the first pulse edge of zero-cross signal which appears immediately after each of the gate signals 14a and 16a rises can be extracted. For example, when a pulse of the gate signal 16a rises at time t.sub.1, only a zero-cross signal pulse edge appearing at time t.sub.2 is extracted and when a pulse of the gate signal 14a rises at time t.sub.3, only a zero-cross signal pulse edge appearing at time t.sub.4 is extracted. Delay lines 12, 13, 15 and 17 are used to place the waveforms in timed relationship. The pulse width can be adjusted by means of a pulse width setting circuit 27.
In the read pulse generator constructed as above, however, the amplitude of non-differentiation signal 7a, 7b occurring in a high density recording region can not extend sufficiently as indicated at 7A in FIG. 3 and is often decreased below a slice level 28 under the influence of a noise and a medium defect as indicated at 7B in FIG. 3. Consequently, a pulse 14A of the gate signal 14a corresponding to the data portion 7A can not be generated, resulting in loss of data as shown at 8A in FIG. 3.